Electric machines are utilized in a wide variety of applications. For example, hybrid/electric vehicles (HEVs) typically include an electric traction drive system that includes an alternating current (AC) electric motor which is driven by a power converter with a direct current (DC) power source, such as a storage battery. Motor windings of the AC electric motor can be coupled to inverter sub-modules of a power inverter module (PIM). Each inverter sub-module includes a pair of switches that switch in a complementary manner to perform a rapid switching function to convert the DC power to AC power. This AC power drives the AC electric motor, which in turn drives a shaft of HEV's drivetrain.
Some traditional HEVs implement two three-phase pulse width modulated (PWM) inverter modules and two three-phase AC machines (e.g., AC motors) each being driven by a corresponding one of the three-phase PWM inverter modules that it is coupled to. In some systems, voltage command signals are applied to a pulse width modulation (PWM) module. The PWM module applies PWM waveforms to the phase voltage command signals to control pulse width modulation of the phase voltage command signals and generate switching signals that are provided to the PWM inverter module.
Many modern high performance AC motor drives use the principle of field oriented control (FOC) or “vector” control to control operation of the AC electric motor. In particular, vector control is often used in variable frequency drives to control the torque applied to the shaft (and thus the speed) of an AC electric motor by controlling the current fed to the AC electric motor. In short, stator phase currents are measured and converted into a corresponding complex space vector. This current vector is then transformed to a coordinate system rotating with the rotor of the AC electric motor.
Recently, researchers have used multi-phase machines in various applications including electric vehicles. As used herein, the term “multi-phase” refers to two or more phases, and can be used to refer to electric machines that have two or more phases. A multi-phase electric machine typically includes a multi-phase PWM inverter module that drives one or more multi-phase AC machine(s). One example of such a multi-phase electric machine is a three-phase AC machine. In a three-phase system, a three-phase PWM inverter module drives one or more three-phase AC machine(s).
In such multi-phase systems, voltage command signals are applied to a pulse width modulation (PWM) module. To control pulse width modulation of the voltage command signals, the PWM module generates PWM waveforms which are equivalent to the incoming voltage command signal in a time averaged sense. The PWM waveforms have a controllable duty cycle with a variable PWM period, and are used to generate switching signals that are provided to the PWM inverter module.
The PWM inverter module uses pulse width modulation (PWM) to generate a variable voltage/frequency output with high efficiency. The PWM voltage waveform is effectively filtered by the motor impedance resulting in a sinusoidal fundamental frequency current going to the motor. However, the PWM inherently results in ripple current generation on the AC output phase currents going to the motor, as well as on the high voltage DC link input. These harmonic currents can in turn create acoustic noise, electromagnetic interference (EMI), bus resonance, or even torque ripple problems. The harmonic currents are typically at the first or second switching frequency carrier group and its sidebands. The harmonic spectrum can have sharp and distinct spikes with large amplitude at the various harmonic frequencies. These large amplitude spikes are often the worst case offenders with respect to the acoustic noise, EMI, bus resonance, and torque ripple.
One conventional method of reducing the amplitude of the distinct harmonics in the current spectrum is to rapidly change the switching frequency of the inverter at a fixed rate. This is known as “dithering.” Dithering has been applied to AC inverters as well as many other types of electrical systems where it is desired to spread the spectrum of emissions due to some periodic cycle (such as system clock or PWM).
Notwithstanding these advances, there are many issues that arise when trying to implement dithering techniques in a practical system.
It would be desirable to provide improved methods, systems and apparatus for implementing dithering in motor drive system that is used to control operation of a multi-phase electric machine. It would also be desirable to provide improved methods, systems and apparatus for computing a PWM voltage advance used in controlling operation of an electric machine. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.